Robert Jervis’s System Effects was published just as systems thinking began to decline among political scientists, who were adopting increasingly strict standards of causal identification, privileging experimental and large-N studies. Many politically consequential system effects are not amenable to research designs that meet these standards, yet they must nonetheless be studied if the most important questions of international politics are to be answered. For example, if nuclear weapons are considered in light of their effect on the international system as a whole, it becomes clear that they have obviated the need for a global balance of power by allowing states to counterbalance threats by acquiring nuclear weapons rather than investing in massive conventional balancing efforts. Similarly, systems thinking should inform our understanding of the impact of a ‘‘unipolar power’’ such as the United States, which has enjoyed an overwhelming preponderance of conventional military power since the fall of the Berlin Wall. A unipolar power is likely to become involved in frequent conflicts because it is not restrained by the presence of a peer competitor.